1. Field of the Invention
This invention relates to a slider mechanism for use in machine tools, measuring instruments and other machines and implements requiring precision linear motion and/or positioning capability and to a method of driving the slider mechanism.
2. Description of the Prior Art
FIG. 14 shows the structure of a small linear slider mechanism 1 of the type conventionally used in machine tools. This slider mechanism, taught by JP-A-HEI 10-166237 and known for its compact size and excellent rigidity, includes a pair of appropriately spaced parallel guide rails 2a, 2b, a first slider (clamp slider) 3 and second slider (main slider) 4 provided as separate units that slide between, and are frictionally retained by, the guide rails 2a and 2b, and a first displacement means (clamp actuator) 5 and second displacement means (feed actuator) 6 for clamping the first slider 3 on the guide rails and feeding the second slider 4 in the longitudinal direction of the guide rails.
Periodic signals differing in phase are applied as displacement commands to the displacement means to produce respective clamping and feeding operations, thereby moving the sliders 3 and 4 by sequential guide rail clamping and feeding operations. The slider moving velocity is changed by changing the phase difference or period of the periodic signals. In the illustrated example, as shown in FIG. 15, the command for the clamp displacement means 5 is a halfsine wave (wave A) and the command for the feed displacement means 6 is a sinewave (wave B). The waves A and B have a certain phase difference.
Slider mechanisms used in miniaturized machine tools, measuring instruments and various other micromachines are required to move smoothly, i.e., with little velocity fluctuation. With the conventional driving method, however, the movement of the slider mechanism is intermittent. Momentary stopping, reversal of movement direction and the like make smooth operation impossible.
FIG. 16 illustrates the movement of the conventional slider mechanism 1. Movement of the second displacement means (feed actuator 6) produces movement of the sliders 3 and 4 only during periods of first displacement means (clamp actuator 5) actuation and the sliders 3 and 4 remain stationary during periods when the clamp actuator 5 is not actuated. In other words, the sliders 3 and 4 move intermittently. Positional error relative to the constant velocity desired in response to the velocity command is therefore great.
The moving velocity of the sliders 3 and 4 can be changed by changing the phase difference of the signals for driving the two displacement means. When this method is adopted and, as shown in FIG. 17(a), a low-velocity command is produced by setting a small phase difference, the amount of overshoot (FIG. 17(b)) becomes large, the sliders 3 and 4 reciprocate back and forth, and the average velocity becomes low. The displacement fluctuation is large and smooth movement cannot be achieved. Moreover, as was pointed out above, the moving velocity does not vary linearly with respect to the applied phase difference.
The method of varying the moving velocity of the sliders 3 and 4 by varying the period of the drive signals applied to the displacement means involves numerous problems. Specifically, when a low-velocity command is issued, the average moving velocity declines but still consists of repeated stops and starts. Since the displacement fluctuation is therefore great, smooth movement cannot be achieved. Moreover, the resulting decrease in the response speed of the velocity control degrades the positional control response.
Another problem is that minute displacements smaller than the per-cycle slider displacement cannot be imparted. In order to utilize the command value to control the slider positions automatically, the control repetition period has to be made greater than the drive signal period. However, changing the drive signal period requires the automatic control period to be varied or greatly increased. This is very inconvenient.
The prior-art method of displacement control thus results in intermittent slider movement, is low in resolution, cannot achieve smooth movement, and is limited in degree of attainable positioning resolution.
This invention was accomplished in light of the foregoing drawbacks of the prior art and has as an object to provide a slider mechanism and a method of driving the slider mechanism that can, without need to vary the drive signal period, achieve smooth forward and reverse movement of the slider mechanism and linear speed control response relative to the speed command.
In order to achieve this object, the present invention provides a slider mechanism comprising two parallel guide rails, a first slider and a second slider constituted as separate members that are frictionally retained by and slidable along the guide rails, a first actuator for driving the first slider to change frictional force between it and the guide rails, a second actuator connecting the first and second sliders and adapted to move the second slider in the longitudinal direction of the guide rails, and drive command means for applying to the first actuator and second actuator a drive command signal that periodically produces a drive operation composed of four sequentially executed drive operation stages each consisting of a linear drive operation.
The invention further provides a method of driving a slider mechanism including two parallel guide rails, a first slider and a second slider constituted as separate members that are frictionally retained by and slidable along the guide rails, a first actuator associated with the first slider, and a second actuator connecting the first and second sliders, which method comprises a step of applying to the first actuator and second actuator a drive command signal that periodically produces a drive operation composed of four sequentially executed drive operation stages each consisting of a linear drive operation for operating the first actuator to change frictional force between the slider mechanism and the guide rails and operating the second actuator to move the second slider in the longitudinal direction of the guide rails.
The moving velocity of the second slider can be changed by varying the drive command signal applied to the second actuator in amplitude.
The means for applying the drive command signal can be means constituted for conducting position control comprising first and second displacement means, displacement detection means for producing a signal proportional to slider displacement, a compensator including at least a proportional element for obtaining an absolute value of a difference signal obtained by subtracting the signal produced by the displacement detection means from a displacement command signal, and a drive waveform generator responsive to the absolute value and xc2x1 sign of the difference signal for changing the shapes of drive command waveforms for the first and second displacement means and the amplitude of the drive command waveform for the second displacement means.
In the present invention, the first and second actuators are thus driven by application of a drive command signal that periodically produces a four-stage drive operation for producing a linear drive operation. The slider mechanism can therefore reliably achieve smooth forward and reverse movement and linear velocity control response to velocity commands.
The above and other objects and features of the present device will become apparent from the following description made with reference to the drawings.